The device includes a housing. The input electrical current circuit contains an optical transmitting unit for producing an optical signal. The optical transmitting unit is arranged in the housing. The output electrical current circuit has an optical receiving unit for receiving the optical signal. The optical receiving unit is arranged in the housing.
The input, and/or output, current circuit(s) are/is contactable via a wire connection.
Optoelectronic devices with an optical transmitting unit and an optical receiving unit are used, for example, for digital and/or analog signal transmission between two electrical current circuits galvanically isolated from one another.
In industrial measurements technology, especially in automation and process control technology, such optoelectronic devices are applied and required in field devices for galvanic isolation of the electrical current circuits for explosion protection. The corresponding field devices ascertain, for example, pressure, flow, fill level, dielectric constant, interface level, temperature or a some other physical and/or chemical, process variable as a process variable in the course of a process. Available from Endress+Hauser are, for example, field devices under the marks, Cerabar, Deltabar, Deltapilot, Promass, Levelflex, Micropilot, Prosonic, Soliphant, Liquiphant, and Easytemp, which serve primarily to determine and/or to monitor at least one of the above referenced process variables of a medium.
Galvanic isolation is applied both in the case of so-called two conductor devices, which transmit the supply energy and the measurement signal via a common line-pair, as well as also in the case of four conductor devices, which have, in each case, a separate line-pair for transmission of the measurement signal and the supply energy. Examples of such field devices with such a device formed as an optocoupler for galvanically isolated electrical current circuits are described in U.S. Pat. No. 4,654,771 A and WO 2004/048905 A1.
Optoelectronic devices used mainly as optocouplers are described, for example, in DE 199 20 403 A1 and U.S. Pat. No. 6,947,620 B2. In general, such optoelectronic devices are composed of at least one transmitting element, e.g. a light-emitting diode, and at least one receiving element, e.g. a photodiode or a phototransistor, with the transmitting element and the receiving element being spatially and galvanically isolated from one another at least via a light conducting element.
In order that such optoelectronic devices used for signal transmission satisfy the explosion protection required in industrial measurements and automation technology, they must also maintain the minimum values required for minimum separations between galvanically isolated, electrical current carrying components as regards air paths, insulation thicknesses and electrical current creep paths. The Ex-i standard IEC60079-11 requires, for example, at a voltage of 375 V a minimal creep path length of 10 mm or a minimum distance of 2 mm under potting compound, or a minimum distance of 1 mm under solid insulation. These distances refer, in such case, in particular, to the minimum distances between the operationally electric current carrying connections and conductive traces of the electrical current circuits coupled by means of such optoelectronic devices. Furthermore, there such optocouplers must also fulfill increased requirements as regards temperature resistance and explosion endangerment risk, as well as also as regards the damages arising in the case of occurring overloads.
In order, in spite of the high safety requirements, to enable a highest possible coupling factor (CTR, current transfer ratio), as well as an, as much as possible, compact form of construction of such optoelectronic devices, their light conducting elements are embodied corresponding to the requirements of explosion protection and signal transmission.
The solutions known from the state of the art weaken either the transferred light signal, since the optical components are e.g. spaced farther from one another, or an additional electrical, or electronic, circuit is necessary, which limits the electrical current flow in the optoelectronic device to the maximum allowable electrical current level, as determined, for example, by a safety certificate. These solutions have, however, an increased space requirement and require the application of additional electrical or electronic components onto the electronics board. If one desires to omit these additional components, then no maximum permissible electrical current level, as given, for example, in a data sheet or in the safety certificate, may be exceeded. In this case, it must, however, be taken into consideration, that the optoelectronic device, in the case of malfunction, can be overloaded with theoretically unlimited electrical current. This overload can, naturally, damage the optoelectronic device. Following the overload, however, a minimum degree of insulation corresponding to an Ex-i standard must remain.
In the case of today's usual semiconductor components in housings with hardened potting compound, an extreme overloading leads, in general, to an explosive-like bursting of the housing and therewith to a wire connection/electrical circuit interruption. Such an uncontrolled bursting of the housing can lead to damaging consequences, not only for the measuring device, but also for the environment of the measuring device.
Known from patent documents EP0434489, U.S. Pat. Nos. 4,107,762, 4,814,946, 6,411,498 are capacitors with embedded, melting fuses, in order, in the case of a defect, for example, in the case of a short circuit between the electrodes of the capacitor, to interrupt the electrical current flow.
Electrical components such as capacitors are, however, not, or only insufficiently, suited for transmission of signals, especially in the case of galvanic isolation, since they are frequency dependent. Additionally, capacitors provide no high degree of insulation between the electrical current circuits, which are to be electrically isolated.